Two methods currently exist to implement depth ranging in turbid media. The first method is known as Low Coherence Interferometry (“LCI”). This method uses a scanning system to vary the reference arm length and acquire the interference signal at a detector and demodulating the fringe pattern to obtain the coherence envelope of the source cross correlation function. Optical coherence tomography (“OCT”) is a means for obtaining a two-dimensional image using LCI. OCT is described by Swanson et al. in U.S. Pat. No. 5,321,501. Multiple variations on OCT have been patented, but many suffer from less than optimal signal to noise ratio (“SNR”), resulting in non-optimal resolution, low imaging frame rates, and poor depth of penetration. Power usage is a factor in such imaging techniques. For example in ophthalmic uses, only a certain number of milliwatts of power is tolerable before thermal damage can occur. Thus, boosting power is not feasible to increase SNR in such environments. It would be desirable to have a method of raising the SNR without appreciably increasing power requirements.
A second method for depth ranging in turbid media is known in the literature as spectral radar. In spectral radar the real part of the cross spectral density of sample and reference arm light is measured with a spectrometer. Depth profile information is encoded on the cross-spectral density modulation. Prior designs for spectral radar is primarily found in the literature.
The use of spectral radar concepts to increase the signal to noise ratio of LCI and OCT have been described earlier. However, in this description, only the real part of the complex spectral density is measured and the method uses a large number of detector elements (about 2,000) to reach scan ranges on the order of a millimeter. It would be desirable to have a method that would allow for an arbitrary number of detector elements. Secondly, the previously described method uses a single charge coupled device (“CCD”) to acquire the data. Since the charge storage capacity is limited, it requires a reduction of the reference arm power to approximately the same level as the sample arm power, giving rise to auto correlation noise on the sample arm light. In addition, since no carrier is generated, the 1/f noise will dominate the noise in this system. Thirdly, even with the short integration times of state of the art CCD technology, phase instabilities in the interferometer reduce fringe visibility of the cross spectral density modulation.